(1) Field of the Invention
This invention relates to a method of producing resin powder and an apparatus therefor, and more particularly, to a method of and an apparatus for continuous production of synthetic resin powder which exhibits a uniform particle size distribution. According to the invention, it is possible to obtain powdered resin with a small average particle size.
Powdered resin, particularly colored powdered resin, has a wide variety of uses including, for example, the easy dispersed pigments, colored plastic pellets, powder coating materials or slurry paints formed from a powdered resin with a particle size of 1 to 80.mu. dispersed in an aqueous medium. Of particular importance is the fact that powder and slurry paints or coating materials contain no organic solvent, and that the powder paints can be recovered for reuse in the painting operation and have in recent years been utilized in ever increasing amounts. On the other hand, the slurry paints are free of any danger of explosion and fire and may be used with any ordinary type of coating apparatus for liquid coatings. It is expected that such slurry paints or coating materials will find a wider range of practical application, and that they will come into greater demand in the future.
(2) Description of the Prior Art
For production of (colored) powdered reesin, there has generally been employed a method in which solid resin as it is or after being mixed with an additive or additives such as pigment, curing agent, etc., by hot melting, is powdered by a mechanical means. However, difficulties are experienced in controlling the powdered particle size and its distribution. In addition, the particles produced are angular in shape and hence the resin particles are poor in physical properties such as their flow characteristics and are inconvenient to handle.
When mixing the additives by hot melting, the following problems were encountered:
(1) The thermosetting resins which are reacted and cured at low temperatures are unusable as they are gelled by the treatment. PA1 (2) Color matching is relatively difficult and time consuming. PA1 (3) As the additives such as pigment, curing agent, etc., cannot be dispersed sufficiently in resin, the coating films or the articles coated by using the colored powdered resin obtained from this method render an unattractive finished surface with insufficient smoothness and distinctness-of-image gloss. PA1 (1) There is a wide difference in shearing force between the central portion of the container and its peripheral portions, so that the particle size distribution of the emulsified or suspended particles is broadened, resulting in resin particles of a non-uniform particle size. PA1 (2) The surface tension is reduced as the solvent elutes the water in the container, causing excessive entrainment of foam by agitation. PA1 (1) Transfer of the solvent into water is extremely retarded or inhibited. PA1 (2) The emulsion droplets or resin particles recombine or unite and thus cause deformation or coarsening of particles. PA1 (3) The amount of water required for solvent extraction becomes dozens of times the theoretical amount. PA1 (a) dissolving a solid resin in a water-miscible solvent to prepare a uniform resin solution; PA1 (b) injecting said resin solution from a nozzle into a water jet streams from another nozzle provided adjacent to the first-said nozzle, said injection of the resin solution being performed directly and simultaneously with the jetting of water, to thereby form fine emulsified droplets of said resin solution in water; PA1 (c) properly adjusting the rates of injection of said resin solution and water from the respective nozzles so that at least a substantial portion of the water-miscible solvent in said emulsion droplets will rapidly transfer into water, thereby producing a suspension of the fine powdered resin; PA1 (d) discharging the thus formed suspension of powdered resin onto a preferably continuously moving endless filter cloth conveyor while sucking and filtering off the liquid phase in the suspension from the backside of said filter cloth conveyor to thereby obtain moistened resin powder; PA1 (e) spraying and washing with water said moist resin powder at the stage following said filtration in the course of movement of said filter cloth conveyor while sucking and filtering off washing water from the backside of said conveyor to form a water containing resin powder, and if need be, further heating said water-containing resin powder at the stage following said washing the course of movement of said filter cloth conveyor while sucking off moisture from the backside of the filter cloth conveyor so that the solid content will exceed 60 by weight %; and PA1 (f) recovering the resin powder from the filter cloth conveyor.
As an improvement over this conventional method, a spray drying technique has been proposed (for example, U.S. Pat. No. 3,561,003). According to this technique, the solution prepared by dissolving solid resin in an organic solvent and, if need be, further adding a pigment is spray-dried after dispersing the pigment (in case it is added) to a predetermined range of particle sizes by a known dispersing machine of the type commonly used in production of coating materials.
This spray drying machine, however, still has some serious disadvantages. For instance, as the resin solution is sprayed into the air and dried, the obtained resin particles prove to be very porous, resulting in reduced bulk specific gravity, due to, for one thing, evaporation of the solvent. In addition, air is trapped in the particle pores, so that when such particles are used for coating, the formed coating film may undergo foaming or expansion to roughen its surface.
Further, for instantaneous drying of the droplets of the resin solution formed by spraying, the droplets must be contacted with hot air of a temperature higher than the boiling point of the solvent at the same time that spraying is conducted. This involves a high danger of fire or explosion in the production process. These problems still remain unsolved.
As a further improvement over these conventional methods, a method has been proposed in which solid resin is dissolved in a water-soluble organic solvent by, if need be, mixing and dispersing a pigment. This resin solution is emulsified in water by spraying, agitation or other means while extracting the organic solvent in the droplets of said resin solution into water. If need be, this is further followed by separation, washing with water or other treatments to obtain non-viscous resin powder (U.S. Pat. Nos. 3,737,401, 3,306,342, 2,379,237 and 2,722,528). (This method is hereinafter referred to as the "wet-type powdering method" for the sake of convenience). Emulsification and extraction in this method may be accomplished either in one step by using all of a predetermined amount of water at one time or in two steps by first performing emulsification with a suitable quantity of water and then extracting the solvent by adding the remaining amount of water. The latter facilitates control of particle size.
In case of practising this method on a relatively small scale, for example on a laboratory scale, a resin solution in a container is dropped, sprayed or poured directly or through a conduit into water in a different cylindrical container, said water being strongly agitated by a common type of agitator having a rotor blade or blades, thereby to emulsify said resin solution in water.
In this case, if water in the container is sufficient, substantially all or the greater part of the solvent in the resin solution is transferred into water to form a solid-state resin powder.
On the other hand, if the amount of water is adjusted to a smaller level, it is possible to obtain an emulsion where the droplets of the resin solution are emulsified in the saturated solution of the solvent (hereinafter referred to simply as "emulsion"). This emulsion is dropped, sprayed or poured into violently agitated water in another container, causing the residual solvent in the droplets of the resin solution to transfer to the water to thereby produce a solid resin powder.
However, in the production of resin powder on an industrial scale through this wet powdering method, it was substantially impossible to control particle size and obtain particles with a globular or almost globular form by mere dimensional enlargement of said cylindrical container equipped with an agitator. In other words, since this method employs a batch technique, there is no alternative but to enlarge the container capacity for mass production of the resin, and this inevitably leads to the following disadvantages:
If the agitator performance is raised in order to solve the problem of (1) above, the phenomenon of (2) is promoted. Further, the foam retarded the propagation of shearing force, and the emulsion droplets deposit on the foam to impare contact with water, causing said droplets to float up as scum on the liquid surface. This gives rise to the following problems:
Therefore, the batch-type apparatuses were necessarily subject to certain limitations with regard to increases in scale and operative power.
A method is also known in which said resin solution (prepared by dissolving solid resin in a water-soluble organic solvent and, if need be, further mixing and dispersing a pigment therein) and water are sprayed separately in a spray chamber so as to capture the droplets of the resin solution in water droplets while extracting the water-soluble organic solvent in the resin solution into water to thereby continuously obtain powdered resin (Japanese Patent Pub. No. 42615/71 and Japanese Patent Laid-Open No. 123770/75). However, as this method makes use of a system in which the droplets of the resin solution once sprayed into the air are again captured, the droplets of the resin solution recombine in the air at a considerable rate to coarsen the particles, so that the obtained resin powder exhibits a large average particle size and a broadened particle size distribution.